1. Field of the Invention
The present invention relates generally to an Orthogonal Frequency Division Multiplexing (OFDM) network, and in particular, to a hybrid forwarding apparatus and method for cooperative relaying using relay terminals.
2. Description of the Related Art
Increased frequency utilization and improved Quality of Service (QoS) performance are requisite for the evolution of mobile communication systems from 2nd Generation (2G) and 3G systems to 4G systems.
A Single-Input Single-Output (SISO) scheme is not practical due to its limited frequency use in fulfilling requirements for data rate and stability.
New transmission strategies are required to satisfy QoS requirements. Multiple antenna techniques, such as a Multiple-Input Multiple-Output (MIMO) scheme, are one of these transmission strategies, which increases system capacity.
MIMO has the benefit of huge spectral efficiency in rich-scattering environments. As long as the number of receive antennas is greater than or equal to that of the transmit antennas, capacity increases linearly with the number of transmit antennas.
MIMO provides multiple signatures of the same transmitted signal to the receiver simultaneously through multiple antennas. Each signature is referred to as a diversity branch in the present invention.
As the diversity branches of the same transmitted signal increases, the probability that all the diversity branches will be in deep fade is reduced, leading to an increase in the reliability at the receiver. However, the MIMO technology requires a plurality of antennas at the transmitter, increasing hardware complexity and cost.
Performance improvements through conventional MIMO techniques may not be achieved all the time because the system performance depends strongly on the number of antennas, scattering environment, spatial fading correlations between transmit antennas and receive antennas, and the distance between network elements.
The above-described advantages of the conventional multiple antenna techniques can be achieved through cooperative communications which create a Virtual Antenna Array (VAA). The VAA is created with the help of relay terminals that assist with the communication of any source-destination pair. The term “cooperative communications” refers to the relaying of communication signals between the source terminal and the destination terminal via the relay terminal. Thus, the concept of the VAA achieves the benefits of multiple antenna techniques produced even in the SISO environment.
The virtual antenna role of the VAA in the source terminal or the destination terminal brings an increase in link reliability and the coverage of the system. As a consequence, the outage probability and Bit Error Rate (BER) of communications between the source terminal and the destination terminal can be reduced.
Furthermore, the VAA increases robustness against environmental changes, thereby increasing data rate. For the same data rate, power consumption is reduced.
One of the primary targets of cooperative communications is to increase the reliability of communications between the source terminal and the destination terminal. To achieve the reliability increase, a forwarding scheme that extracts the diversity offered by the VAA needs to be designed.
“Amplify and Forward (AF)” and “Decode and Forward (DF)” are two main forwarding schemes widely used in relay networks.
A relay terminal employing AF amplifies a received signal, prior to forwarding, whereas a relay terminal employing DF decodes, re-encodes and forwards a received signal.
FIG. 1 is a block diagram of a conventional OFDM relay terminal employing AF. While the following description is made in the context of OFDM, the same is applicable to Time Division Duplex-Code Division Multiple Access (TDD-CDMA) or Time Division Duplex-Time Division Multiple Access (TDD-TDMA).
Referring to FIG. 1, in the OFDM relay terminal, a receiver includes a Radio Frequency (RF) processor 122, an OFDM demodulator 120, and a buffer 124. The OFDM demodulator 120 has an Analog-to-Digital Converter (ADC) module, an OFDM demodulation module, and a decoding module.
The RF processor 122 converts an RF signal received through an antenna to an analog baseband signal.
The ADC module in the OFDM demodulator 120 converts the analog signal received from the RF processor 122 to a digital signal. The OFDM demodulation module converts time-domain sample data received from the ADC module to frequency-domain data by Fast Fourier Transform (FFT). The decoding module decodes the frequency-domain data using a predetermined demodulation method and a predetermined coding rate and stores the decoded data in the buffer 124.
A transmitter of the OFDM relay terminal includes an RF processor 112, an OFDM modulator 110, and an AF module 114. The OFDM modulator 110 has a Digital-to-Analog Converter (DAC) module, an OFDM modulation module, and a coding module.
The AF module 114 performs gain amplification for each subcarrier loaded from the buffer 124. The amount of gain amplification is determined according to the Channel State Information (CSI) of the channel where the relay terminal receives data.
In the OFDM modulator 110, the coding module encodes the gain-amplified data using a coding rate and a modulation scheme corresponding to a Modulation and Coding Scheme (MCS) level. The modulation scheme can be Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), 16-ary Quadrature Amplitude Modulation (16QAM), or 64QAM. The OFDM modulation module converts the coded data to time-domain sample data (i.e. an OFDM symbol) by Inverse Fast Fourier Transform (IFFT). The DAC module converts the sample data to an analog signal.
The RF processor 112 converts the analog signal to an RF signal and transmits the RF signal through the antenna. An RF switch 130 switches the signal received through the antenna to the receiver, and for data transmission, switches the transmitter to the antenna.
FIG. 2 is a block diagram of a conventional OFDM relay employing DF. While the following description is made in the context of OFDM, the same is applicable to TDD-CDMA or TDD-TDMA.
Referring to FIG. 2, in the OFDM relay, a receiver includes the RF processor 122, the OFDM demodulator 120, a detector 240, a decoder 270, and a buffer 280. The OFDM demodulator 120 has an ADC module, an OFDM demodulation module, and a decoding module.
The RF processor 122 converts an RF signal received through an antenna to an analog baseband signal. The ADC module in the OFDM demodulator 120 converts the analog signal received from the RF processor 122 to a digital signal. The OFDM demodulation module converts time-domain sample data received from the ADC module to frequency-domain data by FFT. The decoding module decodes the frequency-domain data using a predetermined demodulation method and a predetermined coding rate.
The detector 240 detects the signal it receives based on the CSI of the channel on which it receives data.
After detection, if the source terminal employs channel coding, the decoder decodes the data received form the detector 240 and stores the decoded data in a buffer 280.
A transmitter of the OFDM relay includes the RF processor 112, the OFDM modulator 110, a subcarrier symbol mapper 230, and an encoder 260. The OFDM modulator 110 has a DAC module, an OFDM modulation module, and a coding module.
The encoder 260 re-encodes data loaded from the buffer 280, if the data was decoded in the decoder 270. The subcarrier symbol mapper 230 maps the re-coded data to subcarriers and provides the mapped data in parallel to the OFDM modulator 110.
In the OFDM modulator 110, the coding module encodes the mapped data using a coding rate and a modulation scheme corresponding to an MCS level. The modulation scheme can be BPSK, QPSK, 16QAM or 64QAM. The OFDM modulation module converts the coded data to time-domain sample data (i.e. an OFDM symbol) by IFFT. The DAC module converts the sample data to an analog signal.
The RF processor 112 converts the analog signal to an RF signal and transmits the RF signal through the antenna. The RF switch 130 switches the signal received through the antenna to the receiver, and for data transmission, switches the transmitter to the antenna.
Coding in the encoder 260 and decoding in the decoder 270 are optional and represented by dotted boxes.
AF is normally affected by noise enhancement at the relay terminal and DF is significantly influenced by the error propagation at the relay terminal. As the OFDM subcarriers undergo independent fading, some subcarriers may provide better results for AF protocol and some other subcarriers may provide better results for DF protocol. Sometimes the direct transmission may provide better results than the relaying with AF and DF.
Thus, there is a need for a technique which selects the AF for those subcarriers which provide better results for AF, select DF for those subcarriers which provide better results for DF and selects the direct transmission for some subcarriers for which it provides better results in a hybrid fashion for a OFDM system over frequency selective fading environment.